10 THE DATA OF BIOLOGY. 



The like is still more conspicuous in other groups of the 

 hydro-carbons, as in the essential oils : sixteen to twenty of 

 which are severally isomeric with essential oil of turpentine. 

 Here the particular kind of molecular mobility implied by 

 these metamorphoses, is well shown : essential oil of turpen- 

 tine being converted into a mixture of several of these poly- 

 merides, by simple exposure to a heat of 460. 



There is one further fact respecting these binary compounds 

 of the four chief organic elements, which must not be over- 

 looked. Those of them which form parts of the living tissues 

 of plants and animals (excluding water which has a me- 

 chanical function, and carbonic acid which is a product of 

 decomposition) are confined to one group the hydro-carbons. 

 And of this group, which is on the average characterized by 

 comparative instability and inertness, these hydro-carbons 

 found in living tissues, are among the most unstable and 

 inert. 



3. Passing now to the substances which contain three 

 of these chief organic elements, we have first to note that 

 along with the greater atomic weight which mostly accom- 

 panies their increased complexity, there is, on the average, 

 a further marked decrease of molecular mobility. Scarcely any 

 of them maintain a gaseous state of ordinary temperatures. 

 One class of them only, the alcohols and their derivatives, 

 evaporate under the usual atmospheric pressure ; but not 

 rapidly unless heated. The fixed oils, though they show that 

 molecular mobility implied by an habitually liquid state, 

 show this in a lower degree than the alcoholic compounds ; 

 and they cannot be reduced to the gaseous state without de- 

 composition. In their allies, the fats, which are solid unless 

 heated, the loss of molecular mobility is still more marked. 

 And throughout the whole series of the fatty acids, in which 

 to a fixed proportion of oxygen there are successively added 

 higher equimultiples of carbon and hydrogen, we see how 

 the molecular mobility decreases with the increasing sizes of 



